Desert

Biology Department

This portion of the greenhouse has a somewhat warmer temperature and is more sunny than the other portions of the museum greenhouse.

Xeric (dry) Environments. These are places where water is scarce. This includes not only deserts, but saline habitats where the high salt content of soils creates water stress, or habitats where the water may be frozen most of the year. Xeric environments may be very localized, such as in rock outcrops at high elevation where the sunlight is intense and water is scanty during the summer. Annual plants growing in xeric environments can avoid drought by completing their life cycles during the brief season when water may be available and then surviving the rest of the year as seeds. Perennial plants which can not avoid the drought in this way, they have evolved a variety of adaptations which allow them to withstand water scarcity. Such plants are known as xerophytes. The following is a partial list of the structural adaptations (modifications that increase fitness) shown by xeric plants. There are interesting physiological adaptations as well including CAM and C4 photosynthetic pathways but we will concentrate on the features you can see in the Bucknell greenhouse.

Stem succulence. Stems that are fleshy and water-storing are referred to as "succulent". Such plants often have no leaves and the stem does all the photosynthesis.

Ribs. Prominent vertical ribs on succulent stems allow them to expand or contract like an accordion as the water content increases or decreases.

Leaf succulence. Plants often store water in fleshy leaves, in which case the stem may be much reduced in size.

Hairiness. A dense covering of hair reflects sunlight, keeping plants cooler - which cuts down water loss. It also reduces water loss by preventing wind from sweeping away the layer of water-saturated air that exists near stomatal openings.

Glaucousness. This term refers to the whitish bloom one sees on many plants. The bloom consists of a fine layer of wax particles that reflects sunlight.

Spines and thorns. These are modified leaves and stems, respectively, and almost certainly evolved in response to predation by thirsty desert animals.

Notice that many of the plants which you might be tempted to call cacti are not in that family (Cactaceae), they are actually genetically unrelated. We have examples of many plant families which show a cactus-like form (read pot labels, remembering that family names end aceae). Compare the large Euphorbia lactea (dragon bones--to the left) or Euphorbia trigona with Idria columnaris (boojum-tree--to the right) and with Cereus or Pereskia (--to the left) cacti. The former evolved in the Old-World deserts while the latter evolved in New-World deserts. While genetically unrelated, these families exhibit convergent evolution in their adaptations to similar xeric conditions. Other examples of convergent evolution in animals include fish and whales.

Look for: "WINDOW PLANTS" or "STONE PLANTS" (on the table to the right).

One way to escape the killing heat of the desert is to go underground, which is what many of the animals do. Window plants (Lithops) are entirely subterranean except the nearly transparent leaf tips which are exposed at the soil surface. These areas of transparency allow light to penetrate to the interior and strike the photosynthetic tissue. But most of the plant is not exposed to the heating effects of high light intensity and to the drying effects of desert air. There are no stomata in the windows, so little water is lost. Growth is slow, but it does occur...so how do these plants remain subterranean? They have contactile roots which continually pull the plants deeper into the ground as the stem elongates. The leaf tips look like stones and so these plants are also known as "stone" plants. They are native to the hottest deserts of southern Africa.

Look for: CRASSULACEAE (a family which has many leaf succulents--scattered throughout the desert area but be sure to see the large Jade tree on the right near the end of the desert section).

This family is interesting for a number of reasons (e.g., presence of the CAM photosynthetic metabolic pathway--a pathway that is very water conservative) One thing you will notice on many species is the apparent absence of a stem. It is there, but stem regions between leaves(internodes) are so short that they can hardly be seen. This produces a "rosette" form.

Study several rosette plants. You should be able to see rather easily that the leaves are arranged so that two sets of spirals can be seen, a clockwise set and a counter-clockwise set. If you were to count the number of spirals in each set, only certain numbers would turn up. They are the numbers in a Fibonacci sequence (1,1,2,3,5,8,13,21,34,55, etc.). Notice that each number is the sum of the two preceding it in the sequence. Normally the pattern cannot be seen because of stem elongation and stem twisting, but it is very obvious in something like a giant sunflower where the seeds are packed into beautiful Fibonacci spirals, or it can easily be seen in a pine cone. In the Compositae (sunflower) family, the disk flowers and seeds are arrranged in two sets of logarithmic spirals, one clockwise, the other anticlockwise which produce arms that radiate from the flower's center.

It is apparent that the shoot tip, where leaves and other structures are born, operates with incredible mathematical precision. Scientists studying these patterns hope to understand the biochemical and physical mechanisms underlying form, but it is a very difficult problem and we are far from understanding it.

Look for: POLLINATION INVOLVING CARRION FLIES AND STAR FLOWERS

If you are in luck, Stapelia spp. (Asclepiadaceae, a family which includes milkweeds) will be in flower. If not, you will have to rely on the drawing in the left hand-margin. The flowers are purplish-red, sort of meat-colored, and you will discover, if it is ripe, that it also smells like rotting meat - more specifically, a long-dead animal. When the flower is mature, it emits foul-smelling gases that attract flies and other carrion-loving organisms which serve as pollinators. They come in search of carrion and are soon "dusted" with pollen as they lay their eggs. They leave to fly to another such plant where, they may be fooled again - but this time their visit will carry out cross pollination because they bring a load of pollen. The larvae that hatch from the eggs die as the flower provides no nourishment.

Aristolochia spp. (Aristolochiaceae, a family which includes wild ginger). The unusual flowers give Aristolochia species their common name, "Dutchman's pipe". Note their color; these flowers operate much like the plant described above - attracting flies and gnats, by producing a foul odor.

Look for: PLANTS IN FLOWER.

The outstanding characteristic of angiosperms or flowering plants is the flower. Most flowers contain two sets of sterile appendages, the sepals and petals, that are attached to the receptacle below the fertile parts of the flower, the stamens and carpels. Given the basic structure of the flower, many variations exist in sex, number of floral parts, arrangement of parts, and symmetry. The Bucknell collection has flowers of many forms that clearly illustrate the great diversity in floral types.

This floral variation relates to the mode of pollination of a given flower. Some flowers are pollinated passively, by the action of the wind or water. The stigma catches pollen grains as they contact its surface. These flowers are not showy as no advertisement of the flowers presence is necessary (e.g., ragweed).

Many flowering plants, however, produce conspicuous, showy blossoms that attract insects and other animals (e.g., goldenrod). These flowers actually direct the activities of the floral visitor so that a high frequency of cross-pollination of the plants will result. In a sense, the angiosperms have transcended their rooted condition and become just as motile, reproductively, as the higher animals.

How could this come about? The more attractive flowers were to early insects (e.g., beetles), the more frequently they would be visited, and the more seeds they would produce. Any chance mutations that made the visits more frequent or more efficient offered an immediate selective advantage. By the beginning of the Cenozoic era (65 million years age), bees, wasps, butterflies, and moths had made their evolutionary debut. The rise of these long-tongued insects, for which flowers are often the only source of nutrition, was a direct result of angiosperm evolution. In turn, insects profoundly influenced the evolutionary course of the angiosperms and contributed greatly to their diversification. This type of evolutionary interaction has been referred to as diffuse coevolution.

Try to predict what the pollinators of the flowers you see might be.

Bee flowers have odors and brightly colored petals, usually blue or yellow. There are often distinctive color patterns (e.g., lines that radiate, circles of varying colors) that make recognition of specific species easier for the bees. For example, many flowers use patterns reflective in the ultraviolet region (unfortunately, our eyes cannot detect ultraviolet). which produce "nectar guides" that aid the bees in locating nectar or pollen resources. As bees collect nectar and/or pollen, they often show a high degree of constancy for certain plant species. This constancy facilitates the efficient handling of flowers but it also increases the likelihood of cross pollination within a plant species. There are at least 20,000 species of bees and about 25,000 species of orchids, and many have an obligatory one-on-one relationship.

Butterfly flowers are also brightly colored and produce odors (although a different range of odors than are found in bee-pollinated flowers). They may be blue and yellow, but many are red or orange. Although their shapes are various, a flat-topped cluster of small flowers is almost invariably a butterfly flower. The flat cluster provides a landing platform. These platforms are displayed to perfection on Asclepias tuberosa, a local plant known as "butterfly weed". Butterfly weed is often covered with butterflies in summer. Flowers with long corolla (i.e., all petals taken together) tubes are frequently butterfly- or moth-flowers since these insects have a long proboscis (i.e., tongue) which they can unroll like a New Year's Eve horn to reach the nectaries at the base of the flower. Moth flowers are open at night and are most commonly white - a color easily seen in the dark.

Beetle flowers. These flowers tend to be either large and solitary, like Calycanthus or Magnolia, or small and aggregated into dense inflorescences (e.g., some Viburnum, Sorbus). Colors are often whitish or dull, and the flowers are open, with easy access to sexual organs and rewards. Odors are often strong and, to a human nose, generally unpleasant. Beetles are considered to be relatively inefficient pollinators due to their smooth, hard exteriors unsuited to adhesion of pollen, their chewing mouthparts, and their ungainly movements.

Bird flowers are usually red and very sturdy. Birds are relatively clumsy (except hummingbirds) and delicate flowers wouldn't stand up to the abuse. These flowers have evolved little or no odor since a bird's sense of smell is very poor. In our part of the world, you may know the red columbine or cardinal flower. Both are bird pollinated. If you have hiked in Rocky Mountain alpine meadows with a red backpack or red hat you have probably been bomb-dived by hummingbirds that check out any red-colored object as a potential food source.

Other pollinators are ants, bats (very common in the tropics) and a variety of small mammals.

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